1. Field of the Invention
The present invention relates to a vertical cavity surface emitting laser diode (hereinafter denoted as VCSEL), in particular, relates to a structure of a mirror defining a cavity of the VCSEL.
2. Related Prior Art
The VCSEL enables to emit light with nearly circular field pattern because the VCSEL emits light from the primary surface of the semiconductor layer, not from an end surface thereof. Moreover, the VCSEL in particular the active portion thereof, may be integrated on single substrate with high density.
The VCSEL provides a lower mirror and an upper mirror on a GaAs substrate, between which an active layer is sandwiched. In one of prior VCSELs, these lower and upper mirrors have a structure of alternately stacked AlAs layer and GaAs layer. These mirrors operate and are called as a distributed Bragg reflector (DBR).
These upper and lower DBRs should have relatively high reflectivity to emit coherent light for the VCSEL. Following two methods are well known to increase the reflectivity: (1) to increase the layers constituting the DBR and (2) to increase the difference of the refractive index of each layer constituting the DBR. However, when semiconductor materials are used to form these two layers in the DBR, taking the lattice constant of two layers adjacent to each other into account, applicable materials are quite restricted. That is, although the substantial difference of the refractive index may be obtained from the single crystal, to grow the semiconductor single crystal in alternate must be necessary to coincide with their lattice constants to each other. Therefore, it is considerably hard to take the latter method to increase the reflectivity. Accordingly, the former method, namely to increase the number of layers, has been an available solution, which increases the size of the VCSEL and complicates the process thereof.
Therefore, one object of the present invention is to provide a vertical cavity surface emitting laser diode having a reflectivity enough to emit coherent light with a reduced number of layers for the upper or the lower DBRs.